Traditionally decorative lights string sets are packed in a box comprising a plurality of folded bases and made of hard paper board or polyron. In the concave space formed by the folded bases, bulb sockets connected in the string set are installed. The packing device of such structure is large in voloume. There is also a waste of material. Furthermore, numbers of folded bases, i.e. numbers of concave spaces are fixed in forming, therefore it can not be adjusted and assembled optionally to comply with various bulb sockets, the packing operation is made very inconvenient.